The invention relates to a mirror. Furthermore, the invention relates to a projection objective comprising such a mirror. Moreover, the invention relates to a projection exposure apparatus for microlithography comprising such a projection objective.
Projection exposure apparatuses for microlithography for the EUV wavelength range have to rely on the assumption that the mirrors used for the imaging of a mask into an image plane have a high reflectivity since, firstly, the product of the reflectivity values of the individual mirrors determines the total transmission of the projection exposure apparatus and since, secondly, the light power of EUV light sources is limited.
Mirrors for the EUV wavelength range around 13 nm having high reflectivity values are known from DE 101 55 711 A1, for example. The mirrors described therein consist of a layer arrangement which is applied on a substrate and which has a sequence of individual layers, wherein the layer arrangement comprises a plurality of surface layer systems each having a periodic sequence of at least two individual layers of different materials that form a period, wherein the number of periods and the thickness of the periods of the individual layer systems decrease from the substrate toward the surface. Such mirrors have a reflectivity of greater than 30% in the case of an angle of incidence interval of between 0° and 20°.
In this case, the angle of incidence is defined as the angle between the direction of incidence of a light ray and the normal to the surface of the mirror at the point where the light ray impinges on the mirror. In this case, the angle of incidence interval results from the angle interval between the largest and the smallest angle of incidence respectively considered for a mirror.
What is disadvantageous about the abovementioned layers, however, is that their reflectivity in the angle of incidence interval specified is not constant, but rather varies. A variation of the reflectivity of a mirror over the angles of incidence is disadvantageous, however, for the use of such a mirror at locations with high angles of incidence and with high angle of incidence changes in a projection objective for microlithography since such a variation leads for example to an excessively large variation of the pupil apodization of such a projection objective. In this case, the pupil apodization is a measure of the intensity fluctuation over the exit pupil of a projection objective.
What is furthermore disadvantageous about the abovementioned layers is that the latter transmit too much EUV radiation to the substrate, as a result of which the substrate is exposed to high doses of EUV radiation over a long period of time. However, under high doses of EUV radiation, substrates for EUV mirrors composed of materials such as for example Zerodur® from Schott AG or ULE® from Corning Inc. tend toward densification of the order of magnitude of a few percent by volume. In the case of generally non-uniform irradiation of the mirrors, said densification leads to a non-uniform change in their surface form, as a result of which the optical imaging properties of the mirrors are changed in an undesirable manner during the operating period.
In order to obtain a high reflectivity of mirrors for the EUV wavelength range it is also necessary to avoid losses on account of stray light, which leads to stringent requirements made of the surface roughness of such mirrors in the so-called HSFR range, see U. Dinger et al. “Mirror substrates for EUV-lithography: progress in metrology and optical fabrication technology” in Proc. SPIE Vol. 4146, 2000, in particular for the definition of the surface roughness in the HSFR range with spatial wavelengths of the roughness of 10 nm to 1 μm (HSFR=high spatial frequency roughness) and in the MSFR range with spatial wavelengths of the roughness of 1 μm to 1 mm (MSFR=mid spatial frequency roughness). Furthermore, such mirrors have to ensure the high reflectivity values and the desired optical imaging quality even over a period of several years under continuous irradiation with high-intensity EUV radiation.
Other mirrors, too, which are used within projection exposure apparatuses for microlithography at wavelengths of less than 250 nm have to have low values for the surface roughness in the HSFR range in order to avoid stray light losses.